(1) Field of the Invention
The present invention relates to a DC-DC converter, and more particularly relates to a DC-DC converter with an output voltage adjusting circuit.
(2) Description of the Prior Art
A conventional power management chip controls a converting circuit transforming an input voltage into a suitable output voltage for driving a load. The power management chip adjusts the output voltage according to a reference voltage, and the reference voltage may be an external signal. For example, in a graphic processing unit (GPU) or a central processing unit (CPU), the system varies the output voltage with a change of the load to save the power. In the situation of the light load, the power management chip controls operation, for example, in a power-saving mode of a diode emulation mode, or even suspend mode. When the reference voltage is adjusted low or the output voltage is suddenly high due to outside interference, the system can't respond in time which causes the power management chip erroneously enters a protection mode.
FIG. 1 is a diagram showing the waveforms in a conventional power management chip. When the power management chip enters into the power-saving mode at a time point t1, a level of a reference voltage is reduced for reducing an output voltage Vout so as to reduce a power consumption of the load (for example, GPU and CPU). Because the load operates under light load, the power management chip enters into the diode emulation mode which turns off the high-side transistor and the low-side transistor and makes the output voltage Vout to keep at a constant value. However, a target voltage Vor corresponding to the reference voltage changes as the reference voltage which makes the power management chip determine the output voltage being over-voltage and execute an over voltage protection at a time point t2.
FIG. 2 is schematic diagram of a power converting circuit in the U.S. Pat. No. 7,855,864. The power converting circuit comprises a control circuit 20, a regulation circuit 22, and a converting circuit. The converting circuit comprises a high-side transistor SW1, a low-side transistor SW2, an inductance L and a capacitance C. The converting circuit is coupled to an input voltage Vin and the converting circuit outputs an output voltage Vout according to control signals S1 and S2 of the regulation circuit 22. A voltage detecting circuit VD is coupled to an output end of the converting circuit for detecting the output voltage Vout and generating a detecting voltage VFB. The control circuit 20 receives a reference voltage Vr and the detecting voltage VFB, and accordingly determines the converting circuit operating in a continuous current mode or a discontinuous current mode and generates a module control signal Smo. The regulation circuit 22 generates the control signals S1 and S2 according to the module control signal Smo and the detecting voltage VFB for respectively controlling the high-side transistor SW1 and the low-side transistor SW2 to stabilize the output voltage Vout.
The control circuit 20 detects the situation of the load to determine whether reverting to the continuous current mode when the power converting circuit operating in the discontinuous current mode. Thereby, when the reference voltage or the output voltage suddenly changes, the power convening circuit can be timely switched to the continuous current mode for adjusting the output voltage. This control way is realized through the mode switching. However, such frequently switch mode will bring the switching loss which is unfavorable to conversion efficiency in light load.